The present invention relates to a roof paver element and a roof paver system including a plurality of interconnected roof paver elements for covering roofs, especially for covering the membranes of single-ply roofing systems.
U.S. Pat. No. 4,655,018, issued to the present inventor, discloses a roof paver element and system for replacing the gravel used as roof ballast to hold down roofing membranes of low-slope roofs against the roof deck when wind conditions may create negative pressures tending to lift the membranes. Each roof paver element includes a generally planar portion having an underside from which extend a plurality of parallel ribs defining drainage grooves. The bottoms of the ribs engage the roof membrane and the top surface of the planar portion includes raised portions which generate air vortexes to break up the smooth flow of air across the roof paver elements, which otherwise would tend to lift the elements. The raised portions also define a safety tread which helps prevent workmen and others on the roof from slipping.
The ends of the grooves are shaped to receive connector members to interconnect the roof paver elements and thereby provide greater resistance against wind uplift. Although the shapes of the grooves permit connectors having a circular cross section, such as sections of tubing, to be used, the roof paver elements disclosed in that patent are more efficiently used in connection with the connector disclosed in U.S. Pat. No. 4,776,144 in the name of the present inventor. The connectors disclosed therein are shaped to provide a substantial area of contact with the portions of the roof paver elements defining the grooves, and flange stops project from the connector members to engage the roof paver elements at the ends of the grooves.
The ribs of the aforesaid roof paver elements define a number of edges, including edges at the bottom which might abrade the roof membrane and thereby cause leakage. Fragments of material also have a tendency to chip off the sharp edges during handling of the roof paver elements. Furthermore, the ribs include sharp linear indentations, and similar linear indentations exist at the junctures of the ribs with the underside of the planar body portion. These sharp indentations give rise to stress concentrations and, thus, define the areas most likely to fail if any failure is to occur.
Insofar as wind uplift performance of paver elements is concerned, the flow of air along the tops of the roof paver elements causes a net pressure differential between the area above the roof paver elements and the space in the drainage grooves below the paver. Up to a certain limit, if the wind velocity increases gradually, this pressure differential tends to be equalized by air transmission through the paver joints. However, if the wind surges suddenly, though briefly, the pressure equalization effect may not have time to occur before the net pressure differential exceeds the gravity load of the pavers, and, thus, the wind lifts the elements out of position, especially where the elements are not interconnected.